A copvporation oe



w. H. PRIESS ,721,503

CONDENSER AND PROCES S July 23, 1929.

Filed April 50. 1920 4 Sheets-Sheet l July 23, 1929. w. H. PRIESS 3CONDENSER AND PROCESS Filed April :50. 1920 4 Sheets-Sheet 2 July 23,1929. w. H. PRIESS CONDENSER AND PROCESS 4 Sheets-Sheet 3 Filed April30. 1920 jar/37% July 23, 1929. w. H. PRIESS I CONDENSER AND PROCESSFiled April 50, 1920 4 Sheets-Sheet 4 a 4 @(JLQ 'b -M' lulu-InfillIIIIII IA IIIIIII Patented July 23, 1929.

WILLIAM: H.

raise CIALTY APPARATUS COMPANY, OF BOSTON, MASSACHUSETTS, A CORPORATIONOF NEW YORK.

CONDENSER AND PROCESS.

Application filed April 30, 1920. Serial No. 377,977.

This application is a continuation in part i of my application SerialNo. 332,421, filed October .42, 1919.

One of the first steps in the p ocess ofmaking condensers consists in.nbling alternate sheets of conducting and dielectric materials to buildup a condenser stack of the required height, these sheets of materialbeing secured together by insulation having suitable properties for thispeculiar class of work. Notwithstanding great care in selecting thesematerials and in assembling them, it is a very common experience to havea con denser break down during test or while in use and it has beendefinitely determined that a very important cause of such break-clownsis the presence of voids, gas, or an excessive amount of insulatingmaterial between the sheets of material that make up the condenser.

It is an important object of the present invention to devise a processof making condensers which will reduce to a minimum imperfections andfaults of the character above mentioned. In other words, the inventionaims to devise a process of building condensers which will eliminate anunnecessary thickness of insulating material between the layers ofconducting and dielectric materials that make up the condenser and whichwill also reduce to a minimum the presence of voids, blisters or bubblesin the condenser. It is also an object of the invention to devise aprocess of making condensers which will produce a condenser that is afirm mechanical mass, that will have a very low dielectric loss and inwhich the leakage between successive conducting elements of thecondenser will be reduced to a minimum. A further object of theinvention is to improve condenser constructions both from an electricaland also from a mechanical standpoint.

The various features of the invention will be readily understood fromthe following description when read in connection with the accompanyingdrawings, in which Figure l'is a View, partly in vertical cross sectionand partly in side elevation, showing the more important parts of amachine for coating the dielectric elements;

Fig. 2 is a side elevation showing the construction of a condenserstack;

Fig. 8 is a front elevation of a baking stand;

Figs. and 5 are plan. and. side views, re

spectively, of the baking stand shown in Fig. 3;

Fig. 6 is a plan view, partly in cross section, of another form ofbaking stand;

Fig. 7 is a plan view of a condenser after it has been baked;

Figs. 8 and 9 are vertical cross sectional views at right angles to eachother of a condenser after it has been baked, and showing the elementsenlarged;

- Fig. 10 is a perspective View of a finished condenser Figs. 11 and 12are plan and side views, respectively, of a baking clamp;

Fig. 13 is a view similar to Fig. 12 but shows a condenser secured in,the clamp;

Fl l.-lisa front elevation showing a step in the process ofmanufacturing of a modified construction of condenser, and the apparatusused;

Figs. 15 and 16 are side and plan views of the construction shown inFig. 14;; and

l? and 18' are plan. and side views, a holy, of the finished condenser.

Assuming that the mica or other dielectric elements to be used in thecondenser have been made by splitting the mica sheets to the desiredthickness and cutting them out to the required shape and dimensions,andalso that the tin foil or other conducting elements of suitable shapeand size have been provided, the next step is to coat the mica elementswith vari'iisli or other adhesive insulating material. it is desirablethat this coating shall be very thin, and that it shall completely coverboth surfaces of each mica sheet. This coating operation may beperformed in any suitable manner and with any suitable insulatingmaterial, but in order to obtain the desirable results ust mentioned Iprefer to perform the operation in the coating machine illustrated inFig. 1.

This machine comprises upper and lower rolls 2 and 3, respectively,mounted in suitable bearings and geared together. The rolls areyieldingly forced together by springs and he lower roll dips in a bodyof varnish or other adhesive material in a dip pan t. Varnish issupplied to this clip pan from a suit able tank 5 and the overflow iscaught in an outer pan or reservoir 6 and conducted by a pipe 7 to atank 8 where it is saved; The surfaces of the rolls at opposite sides oftheir act ve areas are grooved to prevent the var- .nislt from. worldinto the l rings. In

llD

using this machine the mica sheets are fed between the rolls and thuseach receives a very thin but substantially uniform coating of varnishcovering its entire area. -Any one of a great variety of insulatingvarnishes may be used, but I prefer to use a copal varnish containing abenzol or similar solvent.

The mica films ejected from between the rolls drop on to a conveyor belt10 which delivers them to a hopper 11 through which a current of air isblown by a fan 12. Due to the fact that the coatings of varnish are verythin and that the solvent is very volatile, the mica films dry 'veryquickly and the length andrate of travel of the conveyor 10 are madesuch that the mica films are sufficiently dry by the time they aredelivered to the hopper 11 so that they do not stick together. The fan12 also exerts a further drying action on the lilms so that the greaterpart of the solvent in the coating is eliminated before the films aredeposited in the hopper. Both the rolls and the conveyor are driven froma motor 13 by connect-ions readily understood from an inspection of thedrawings.

After the dielectric elements have been coated and partially dried, asabove described, they are next assembled with sheets of tin foil, orother conducting material of the proper size and shape, to form acondenser stack. The condenser stack may either be built up in the formof a tall stack that later is split up into units of the requiredheight, or the stack may be built up in the form of individual units,each unit stack being kept separate from the others. Fig. 2 illustratesdiagrammatically a portion of a stack, the two sets of foil elementsbeing indicated. at C and D, respectively, and the mica elementsseparating them at M. The elements are here shown separated from eachother for the sake of clearness.

After the assembly of the stack has been completed, the stack is nextheated as by baking for the purpose of driving out the solvent in thevarnish, eliminating any excess of varnish between the elements, anddriving out any gas or vapor that may be pocketed between the elementsof the stack. A further result accomplished in the baking operation whencertain varnishes are used is the oxidizing or chemical changing of thesolid constituents of the varnish. For this purpose the condenser stackpreferably is transferred to a baking stand of the general characterillustrated in Figs. 3, 4t and 5.

Assuming that the condenser stack has been assembled in units, as abovedescribed. those units indicated at S are placed one above the other ina stand whichcomprises a base 16 and two posts 17 and 18 rising from theopposite sides of the base and threaded near their upper ends. Theindividualunits S are separated from each other by separator strips 19of brass or other suitable material having holes drilled in them so thatthey slide freely on the posts 17 and 18.

For the purpose of clamping this stack in the stand and compressing thestack, a pressure plate 20 is placed on top of the stack, a. springplate 21 is superposed on this pressure plate and another heavier plate22 is placed on top of the spring plate 21. Nuts 2323 are then threaded011 the posts 17 and 18 and are tightened up to apply pressure throughthe plate 22 and spring plate 21 to the entire stack. The spring plate21 preferably con sists of a steel plate bowed or sprung, as best shownin Fig. 3, and of such thickness that the proper pressures are obtained.A pressure of approximately 600 pounds per square inch of condenser isordinarily used. Usually the nuts 2323 are turned down before the bakingoperation is begun until the spring plate is practically flat. Thebaking stand with the stack of condenser units clamped therein is thenplaced in an oven and baked for several hours at a temperature ofapproximately 110115 C.

The condenser shown in the baking stand in Figs. 3, 4 and 5 is of asomewhat different form from that shown in Figs. 2 and 7, since theformer type of condenser is provided with holes 25 extending verticallytherethrough which later receive pins by which the finished condenser issecured to its support. In bak-,

ing this type of condenser separator plates 19 are used which aredrilled to receive pins 26 that extend through the holes 25 25 in thecondenser. These pins are held in position by friction and as the stackis compressed the pins are forced downwardly by their contact with thespring plate 21 through holes provided for them in the base 16, as shownin Figs. 3 and 5. These pins serve a useful purpose in the lakingoperation since they support the elements of the condenser againstlateral slip or displacement.

\Vith the type of condenser shown in Figs. 2 and 7, I prefer to use abaking stand in which four posts 28 of the cross sectional shape-shownin Fig. 6 are located, respectively, at the four corners of thestack,the posts being grooved longitudinally to receive the corners of themica sheets M. Pressure is applied to the stack in the same way as inthe construction previously described but the engagement of the posts 28with the corners of the mica sheets support these sheets against lateralslip or displacement.

lVhile, as above stated, the greater part of the solvent in the varnishcoating of the dielectric elements is eliminated before these elementsare assembled to form a condenser stack, a certain amount of solventstill remains in the lower layers of the varnish coating. Furthermore,the coating is thicker than is necessary. The baking of the stack whileit is maintained under a heavy spring pressure squeezes out the surplusvarnish which softens under the relatively high degree of heat to whichit is subjected. 'At the same'time this comparatively high heatvaporizes the solvent together with any other vaporizable elements, suchfor instance as traces of water, which may be pocketed between thesheets, and the heat, together with the pressure, cooperate to reduce toa mini mum gas, vapor, and the thickness of the adhesive remainingbetween the sheets. In other words, the heat tends to diltuse any gasesor vaporizable materials present between the sheets while the pressuretends to squeeze out these materials from between the sheets of thestack. lVhen separator plates 19 made of brass or similar material areused, as dis tinguished from insulating material, they serve not only toseparate the units and give the stack additional mechanical rigidity,but also to conduct heat into the interior of the stack and thus tofacilitate the baking operation. The even is ventilated during thebaking operation for the purpose of carrying oil the solvent vapors andalso, when an oxidizable varnish such as copal varnish is used, toprovide an ample supply of oxygen to eficct this oxidation.

As the baking operation progresses there is a tendency to relieve thepressure on thestack due to the expansion of the rods 17 and 18 and alsodue to a reduction in the height of the stack caused by the eliminationof excess adhesive materials from between the elements of the stack.This tendency is countor-acted to a substantial degree by the springplate 21. I find that better results, however, can be obtained bytightening up the clamp as the baking operation progresses to more orless restore the initial compression. For instance, in one type ofcondenser manufactured according to this process, the baking operationis conducted at about 110l15 C. and requires a period of approximatelytwelve hours. At the end of the first three hours, and again at the endof nine hours, the baking stand is removed from the oven long enough totighten up the nuts E2323 and these nuts are tightened up a third-timewhen the baking operation has been completed and immediately after thebaking stand is removed from the oven, and, consequently, while thestack is still at baking temperature. In other cases only onereadjustment of the pressure is made and in such cases this readjustment usually made at substantially the end of the baking operation.It will, of course, be understood that the temperature at which thebaking operation is conducted, the duration otthis operation, and thereadjusting of the pressure, will depend upon the nature of theadhensivematerial used in the condenser, the nature of the service for which thecondenser is designed, and other considerations. I

I prefer to use a dielectric in the condenser of the above describedprocess which will form a condenser of superior electrical qualities. Ifind mica the best for his purpose. Mica is also non-porous and alsonon-inflammable. Hence under compression and baking, the adhesive, whichis a poor dielectric, does not penetrate the dielectric as in fibrousdielectrics, nor is there danger of carbonizing the dielectric whenbaked. as in dielectrics of organic composition. Upon compression andbaking, the greater part of the adhesive is forced into the marginswhere it serves as a protector against dirt and moisture and to a verysmall degree only forming part of the dielectric, sutlicient only tohold the sheets together.

Aft-er the baking operation has been completed and the pressure on thestack has been restored, above described, immediately fol lowing theremoval of the baking stand from the oven, the rods 26, if used, arewithdrawn and the stack is allowed to cool while still maintained underspring pressure. Usually this cooling action is hastened by directing ablast of air against the projecting ends of: the toil elements. lVhenthe temperature of the stack has been reduced to substantially that ofthe room, the pressure on the stack is relieved and the condenser unitsare removed from the bakin stand. It the separator plates 19 have notbeen used but the stack has simply been baked as one tall unit,'thisunit or stack is next split up into units of the desired height. I

I regard the use of a spring in the application of pressure asespecially important. The very desirable results produced by thisprocess apparently are due to a more thorough elimination of gas andvapor from between the elements of the condenser and to a reduction inthe thickness of the varnish or other adhesive remaining between thecondenser elements after the baking operation has been completed.

Another factor contributing to the superior results produced by thisprocess is the elimination of the greater part o1 the solvent from thevarnish coating on the dielectric elements before these elements areassembled to form a condenser stack. This leaves only enough solvent inthe coating to cause the coating to soften and become tacky during thebaking operation, which result is desirable at this time for the purposeof adhesively securing all the sheets of material firmly together, andsince only a relatively small quantity of solventstill remains in thecoating, the elimination of the solvent can be made much more completethan would otherwise be possible. A further advantageous result producedby this process is the sealing of the marginal spaces between adjacentdielectric elements. It will be seen from an inspection of Fig. 7 thatthe part of each conducting element lying on a mica sheet is surroundedon three sides by an inactive margin of mica; that is, an area of micathat is not subjected to the dielectric strain. This margin is coveredwith varnish during the coating operation, as above described, and thecompression to which the condenser stack is subjected during the bakingoperation results in squeezing additional varnish out from between theoverlapping sheet and into this marginal area.

This varnish completely seals the space between the inactive margins ofadjacent dielectric elements, as indicated at E, Figs. 8 and 9. It willbe understood that when a condenser is in use there is a tendency forthe current to discharge from the edge of one of the conducting elementsacross the inactive margin to the edge of the next adjacent conductingelement. Ordinarily a comparatively free path is left for this dischargeover the surface of the mica. The sealing of these marginal spaces,however, by varnish introduces a solid wall of insulating material inthe path of this discharge and thus reduces very substantially thesurface leakage that otherwise would be present in the condenser.

In other words, this construction reduces the electrical loss in thecondenser.

After the baking operation has been completed, the condenser will befound to be a firm, mechanical mass capable of withstanding very roughusage. The sealing of the edges, as above described, contributes to thisresult and also is of advantage when the condenser is to be used in adamp location.

After the condenser has been baked its manufacture may be completed inany desired manner. According to the process which I prefer at present,the condenser units are next tested and any units which fail towithstand the tests are discarded. The units which sucsessfuly comethrough the testing operation, however, next have the projecting ends offoil trimmed off and suitable clips, such for instance as thoseindicated at and 31in Fig. 10, are secured to the opposite edges of thecondenser in contact with the respective sets of foil elements. In orderto reduce still further the electrical losses in the condenser and alsofor the purpose of improving its mechanical structure, it is preferableunder some conditions to repeat the baking operation. Before this bakingis begun, however, I prefer to coat the edges of the condenser sheetswith varnish in order to seal any cracks in it or to check any tendencyto split that may have been started by the clipping opera tion. Thissecond baking need not be continued as long as the first-but usually isfrom six to eight hours at a temperature from 110 to 115 C. Theexactnatureof the change produced in the condenser by this second baking isdifficult to determine but probably is due to some change produced inthe exceedingly thin film of varnish that still remains between theadjacent elements of the condenser. 1

have determined, however, by actual test that the electricalcharacteristics of the condenser are improved by this second baking andthat they can be still further improved by a third or fourth baking. Thedegree of improvement, however, decreases with successive bakings andusually it is not advisable from a manufacturing standpoint to carry thecondenser through more than two of these heat cycles. As shown in Fig.10, the sheet metal clips or clamps 30, 31 are formed with integrallyprojecting terminal connectors, which are preferably perforated as shownto facilitate the making of circuit connections with the completedcondenser, such connectors projecting away from the stack, as shown.

For the purpose of compressing the condenser stack during the baking ofthe condenser after the clips have been secured to the condenser, Iprefer to use a novel form of baking clamp illustrated in Figs. 11, 12and 13. This clamp comprises a base 33 having a threaded post or pin 34secured thereto at one end. Another screw threaded post 35 is secured tothe opposite end of the base by a pivot 36. A spring plate 37 has a holethrough one end to fit loosely on the pin 34 and is notched at itsopposite end to receive the swinging pin 35, the upward movement of thisspring on the post 34 being limited by a nut 38 threaded on this postand adjustably secured in position by a setscrew 39. The condenser isclamped between this spring plate 37 and the base 30 by pressure exertedon the spring through a nut 40 threaded on the pin 35. In order toenable this apparatus to exert pressure uniformly over the entiresurface of the condenser, a plate 41 is reinovably secured to the uppersurface of the base 33 by means of a screw 42, this plate being shapedto fit the lower face of the condenser between the clips. A similarplate 43 is secured tothelower face of the spring 37, this plate beingprovided with a locating pin 44 and a threaded stud 45 that projectthrough the spring plate and being held in position by a nut 46 threadedon the stud 45. This plate fits the upper surface of the condenserbetween the clips.

The apparatus is very convenient for use in baking condensers after theclips have been applied to them whether such baking is for the purposeof improving the electrical characteristics of the condenser or is forthe purpose of baking on a. label or cover slip, or for any otherreason. It will be seen that after the nut 38 has been adjusted at theproper height to take a condenser of a given thickness, it isunnecessary to change the adjustment of this nut again until anothersize of condenser is to be baked. The condenser is slipped between theplates 41 and 4", the pin 35 is swung into its upright posit-ion. asshown in Fig. 12, and the nut 40 is quickly turned down to straighten.spring plate 37, as

shown in Fig. 18, thus applying a fairly heavy pressure to thecondenser. After the baking operation has been completed the condensercan be quickly removed by reversing the ope *ations above described. Afurther advantage of this construction is that the plates 41 and 43 canbe quickly changed to accommodate different sizes and styles ofcondensers.

While the foregoing description has referred to the coating of thedielectric elements of the condenser, it would be possible to coat theconducting elements instead or to coat both sets of elements wit-hanin'sulating material more or less adhesive. In some cases, however, itis not practicable to coat the conducting elements because they are toothin and flimsy and it is necessary to coat one set only of theseelements. It is not regarded, therefore, as a departure from thisinvention to coat the other elements of the condenser if desired, nor isit regarded as a. departure to use an adhesive material other thanvarnish, this material being mentioned specifically be cause it ispreferred for the particular type of condenser specifically describedherein.

A somewhat different form of the invention is shown in Figs. 17 and 18and certain steps in its process of manufacture and in the apparatuspreferably used in this process are illustrated in Figs. 14, 15 and 16.

In this form of the invention the coating and assembling operations areperformed in the same manner as above described. Any desired number ofthe units S are then placed in a baking stand having parts 16, 17 and 18like the baking stand shown in Figs. 3 and 4. In this arrangement,however, each unit S is located between two metallic spring platesnormally convexly curved or bowed with reference to the condenser, asclearly shown in Fig. 15, one of these plates being designated at andthe other at 51. It will be seen that the plates are alike inconstruction and that each plate is provided at opposite ends withprojections extending at substantially right angles to the plate anddesigned to fit over the opposite sides or edges of the condenser unit.Ears 52 and 53 also project on these plates and have holes punched inthem. The parts 50 and 51 consist preferably of a metal which can beeasily bent or shaped to form the intermediate spring portions and therigid angular projections. Any suitable sheet metal is adapted for thispurpose, such as steel or phosphor bronze. In assembling the parts 50and 51, the spring portions are arranged on opposite sides of the stackwith their convex surfaces facing the same and in parallelism with thesheets composing the stack. They are then compressed. flat against thestack and rigidly secured at opposite sides as hereinafter described tomaintain the sheets together under spring compression.

prises two parts which can be cheaply made and easily assembled tomaintain the stack under good compression and at the same time allow theleads to be brought out at opposite ends of the stack, the stackprojecting at opposite ends beyond the clamp. The individual units withtheir respective pairs of plates are separated by plates 19, the entirestack of units being set up in the baking stand, as shown in Fig. 15.This baking stand also is provided with a pressure plate 22 mounted toslide on the rods 17 and 18 and arranged to bear on the top of the stackof units. Any suitable means may be provided to compress the stack, thatshown consisting of a square threaded screw 54 having a ball endedconnection with the pressure plate 22 at its lower end and threadedthrough a yoke or plate 55 which is removably secured to the upper endsof the rods 17 and 18. The hands 56 is secured to this screw and affordsa convenient means for turning it. It will be seen from an inspection ofFig. 14 that with this arrangement the entire stack can be compresseduntil the springs 50 and 51 are straightened out, thus compressing theunits S with a spring pressure in the same way that pressure is appliedto them in the arrangement shown in Figs. 3 and 4.

The heating or baking, and if desired also, the re-heating or subsequentbaking operations are performed in the same manner as above described,but after the baking oper ation has been completed the projecting endsof the spring plates which have now been brought into overlappingrelationship at opposite sides of each unit, are permanently securedtogether by soldering or welding. Preferably this securing operation isperformed by spot welding, as indicated at 57, Fig. 18. The condenserterminals 58, 59 are shown in Figs. 17 and 18 connected to thefoil-sheets projecting from the stack. As shown in Fig. 17, the integralparts 52, 53, formed with holes, constitute means for securing thefinished condenser in place.

This process results in enclosing the condenser stack in a casing whichbecause of its resilient nature and the manner in which it is secured onthe stack, exerts a permanent spring pressure on the stack oversubstantially its entire active area. After the securing operation theunits may be removed from the baking stand.

The connections to the opposite sets of conducting elements can be madein any convenient manner as, for instance, by soldering leads 58 and 59to these conducting elements, respectively, with a suitable soft solder.The punched cars 52 and 53 are provided to facilitate the securing ofthe completed condenser to its support. This construction provides avery compact and efficient condenser especially adapted for locationswhere the condenser is subjected to rough usage.

The expressions mica and foil sheets are used herein to includesubstantially equivalent materials, both being substantially nonporousand non-impregnatable, and uncompressed under even the high pressureemployed in accordance with the invention.

hat is claimed as new is:

1. That improvement in the process of making electrical condensers whichconsists in applying a thin film of varnish to substantially all partsof both sides of sheets of dielectric material, drying said varnish moreor less, then building said sheets into a stack with sheets of conductorplaced between successive sheets of dielectric, compressing said stackin the direction of its height under resilient pressure, and baking saidstack while under said resilient pressure.

2. That improvement in the process of making electrical condensers whichconsists in coating both sides of the dielectric elements with a film ofvarnish covering substantially all parts thereof, and of substantiallyuniform thickness, drying said varnish more or less, building saidsheets into a stack with conducting sheets interposed between suecessivesheets of dielectric, applying spring pressure to saidstack in thedirection of its height, baking the same while under said springpressure, and maintaining ventilation in the baking chamber to supplyoxygen for chemical changes in the varnish and. to carry off evaporatedsolvent.

3. That improvement in the process of making electrical condensers whichcomprises the steps of coating the dielectric elements of the condenserswith varnish, drying the coated elements sufficiently to eliminate thegreater part of the solvent in the coating, assembling said elementswith conducting elements to form a condenser stack, compressing saidstack, baking the stack while so compressed, adjusting the pressure onthe stack during the baking operation, and maintaining said stack undercompression while it cools.

4. That improvement in the process of making electrical condensers whichcomprises the steps of covering the opposite sides of each dielectricelement of a condenser with a thin coating of varnish, drying the coatedelements sufliciently to enable them to be conveniently handled,assembling said elements with conducting elements to form a condenserstack, compressing said stack in the direction of its height with aresilient pressure, baking the stack for several hours at a temperatureof over 100 C. while the stack is so compressed, adjusting the pressureon the stack after the baking operation has progressed several hours andwhile the stack is .at baking temperature, and maintaining pressure onthe stack while it cools.

5. That improvement in the process of making electrical condensers whichconsists in placing a condenser stack between two spring plates thatengage the opposite ends of said stack, said plates normally tending toassume a convexly curved shape with refercnce to the stack and havingparts adapted to be brought into engagement at opposite sides of thestack, applying pressure throu h said plates to compress the stackthereby straightening the plates against the ends of the stack andbringing said parts into engagement, and then fusing said parts togetherwhile the plates hold the stack so compressed.

6. The hereindescribed process of making electrical condensers whichconsists in applying thin uniform coatings of insulating resinousmaterial over both sides of mica sheets, at least partially drying saidsheets, building said sheets into a stack with conducting sheets andthen heating said stack for several hours at a temperature of the orderof 100 C. and under a pressure of the order of hundreds of pounds persquare inch to modify the physical characteristics of the material.

7. The improvement in the art of making mica and foil sheet condensers,which consists in coating the mica sheets with a more or less adhesiveinsulating liquid; drying the coatings on such mica sheets more or lessat ordinary temperature; assembling such mica sheets with the foilsheets in a stack; applying a high mechanical compression to the facesof the stack; subjecting the compressed stack for a substantial time toa sufliciently high temperature under oxidizing conditions to soften thecoatings between the micas and foil sheets and to vaporize more or lessof the solvent in said coatings; said compression being suiticientlyhigh to force out excess softened coatings from between the foil sheets,and thereby reduce the length of the stack; and more or less restoringsaid high compression when the coatings of reduced thickness are insoftened condition.

8. The improvement in the art of making mica and foil sheet condensers,which consists in coating the mica sheets with a more or less adhesiveinsulating liquid; drying the coatings on such mica sheets more or lessat ordinary temperature; assembling such mica sheets with the foilsheets in a stack; applying a high mechanical compression via a springplate to the faces of the stack; subjecting the compressed stack for asubstantial time to a sufficiently high temperature to soften thecoatings between the micas and foil sheets and to vaporize more or lessof the solvent in said coatings; said spring plate compression beingsufficiently high to force out excess softened coatings from between thefoil sheets and therebyreduce the length of the stack; and more or lessrestoring the compressing flexing of said spring plate when the coatingsof reduced thickness are in softened condition while subjecting theinsulating material to taining sufficient pressure on the faces of thestack during said heat treatment to force out excess softened coatingsfrom between the mieas and foil sheets; applying a metal clip to thestack; coating the edges of the stack sheets with insulating liquid; andsubjecting the clipped and edge-coated stacks to heat treatment undertemperature conditions similar to those of said first heat treatment.

In testimony whereof I have signed my name to this specification.

lVILLIAM H. PRIESS,

